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iPhone Duo Battery Life Depends on Which Screen You Use
The iPhone Duo has what Apple is calling an "iPhone-first dual battery system" with one battery on each side of the device. Apple optimized physical space for battery capacity to make sure that the battery lasts as long as possible. When using the larger 7.6-inch inner display (unfolded), the battery lasts for up to 31 hours of video playback. If you're using just the 5.4-inch outer display, the battery lasts for up to 44 hours when watching video.
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